The present invention relates to a semiconductor device, and more particularly, it relates to a semiconductor device including a field effect transistor using a nitride semiconductor and having a protection diode.
A group III-V nitride semiconductor is a compound semiconductor including aluminum (Al), boron (B), gallium (Ga) or indium (In) and nitrogen (N) and represented by a general formula, BwAlxGayInzN (wherein w+x+y+z=1; and 0≦w, x, y, z≦1).
A nitride semiconductor has various advantages such as a large band gap and a resultant high breakdown voltage, high electron saturation velocity, high electron mobility and a high electron density on a heterojunction, and therefore, it has been earnestly studied and developed for application to a short-wavelength light emitting device, a high-output high-frequency device, a high-frequency low-noise amplifying device or the like.
In particular, a heterojunction structure in which group III-V nitride semiconductor layers respectively having different composition ratios of group III-V elements and having different band gaps are stacked, or a quantum well structure or a superlattice structure in which a plurality of such group III-V nitride semiconductor layers are stacked is used as a basic structure of the aforementioned devices because the modulation degree of an electron density can be controlled within the device by employing such a structure.
One of semiconductor elements using the heterojunction structure is a heterojunction field effect transistor (HFET). An HFET is a rapidly operating element and is expected to be applied to a high-output device, a power switching device, a high-frequency power device, a high-frequency low-noise amplifier or the like.
Semiconductor devices are required to be reduced in size for faster operation, and an HFET used in such application is not an exception. An HFET using a nitride semiconductor has, however, restriction in surge resistance of a gate, and hence, there is a limit in the refinement of the gate. Also, in the application to a power switching device or the like, the surge resistance is required to be further improved.
In order to improve the surge resistance of an element in a conventional semiconductor device, a method in which a protection element is formed separately from the element is known (see, for example, Japanese Laid-Open Patent Publication No. 60-10653). Also, a protection circuit is conventionally formed as an external circuit.
However, in the conventional semiconductor device, since the protection circuit is externally formed or the protection element is separately formed for protecting the gate from a surge voltage, the area of the whole circuit or the element is disadvantageously increased. Also, in the case where the protection element is separately formed, it is necessary to perform a procedure for forming a diffusion layer to be formed into the protection element, which disadvantageously makes the fabrication complicated.